In S/O suspensions where nano-sized microparticles of a water-soluble substance are dispersed in oil, the water-soluble substance can be dispersed stably in an oil phase by causing a surfactant to coexist with the microparticles. Therefore, in a case where such a suspension is used in an external preparation for the skin, for instance, absorption of the water-soluble substance on the skin surface, which is hydrophobic, can be increased.
Accordingly, recent years have witnessed ongoing development of pharmaceuticals, quasi-drugs, cosmetics and so forth of S/O suspension type. Similarly, pharmaceuticals, quasi-drugs, cosmetics and the like of S/O/W emulsion-type have been proposed in which an S/O suspension is further dispersed in water.
The applicants of the present application have already patented a production method such as the following one, as a method for producing such an S/O suspension. Specifically, known methods for producing an S/O suspension include methods that are characterized by thermally dewatering or vacuum-dewatering a W/O emulsion, to produce as a result an S/O suspension in which a water-soluble solid substance, in the form of particles having an average particle size of 20 nm to 10 μm, are dispersed in an oil phase (Patent Document 1)